1. Field of the Invention
The present invention relates to a linearized differential amplifier which can provide a constant transconductance in a wide range.
2. Description of the Prior Art
Emitter coupled pairs which are compositional elements of differential amplifiers are widely used as a basic building block for amplifiers such as an amplifier for an initial stage of an operation amplifier.
FIG. 1 is a diagram to explain an ordinary emitter coupled pair, i.e., a differential pair. In the same diagram, reference numerals 1 and 2 show input terminals, 3 and 4 show output terminals and 5 and 6 denote a first and a second bipolar transistor respectively composing the differential pair. Moreover, 7 and 8 respectively designate dc (direct current) power sources for supplying a first and a second offset voltage, 9 shows a dc current source for determining an operation current for the differential pair, 10 shows a positive power source line, 11 shows a negative power source line, 12 and 13 show load current sources and 14 shows a load resistance. Incidentally, reference numeral 16 shows a portion as the whole differential pair except a load portion 15.
In FIG. 1, when a current value of the current source 9 is I.sub.EE, a voltage value of the voltage sources 7 and 8 for giving offset voltage is zero, current values of the load current sources 12 and 13 are respectively I.sub.EE /2, and an input voltage to be applied between the input terminals 1 and 2 is V.sub.d, further, .alpha..sub.F is a common base forward current gain of the transistors 7 and 8, and V.sub.T designates a thermal voltage, a current I.sub.d flowing in the load resistance 14 is expressed by the following equation (1) as described in literature [A]. EQU I.sub.d =.alpha..sub.F .multidot.tan h (-V.sub.d /V.sub.T) (1)
Literature [A]: Paul R. Gray and Robert G. Meyer; "Analysis and Design of Analog Integrated Circuits" second edition, pp. 194-197, John Wiley & Sons, Inc., New York, 1984.
As shown in FIG. 2A, in the relation between the input voltage V.sub.d and the output current I.sub.d in the equation (1), I.sub.d changes linearly in proportion to V.sub.d when an absolute value of V.sub.d is small. However, when the absolute value of V.sub.d becomes large, I.sub.d becomes far from the linear change and approaches .+-.I.sub.EE gradually.
To see the linear range, it is convenient to investigate a curve of transconductance G.sub.m obtained by differentiating I.sub.d by V.sub.d, that is, expressed by the following equation (2). EQU G.sub.m (V.sub.d)=(.alpha..sub.F .multidot.I.sub.EE)/2 V.sub.T .multidot.[1-tan h.sup.2 (-V.sub.d /2 V.sub.T)] (2)
The curve of G.sub.m is in the shape of a symmetrical bell as shown in FIG. 2B.
For simplification, the following normalization (3) will be used hereinafter. ##EQU1##
Moreover, the equations (1) and (2) will be normalized as follows in the following explanation. EQU y=tan h (x) (4) EQU G.sub.m (x)=dy/dx=1-tan h.sup.2 (x) (5)
Generally, an operational amplifier is used with negative feedback and has a large gain in the second stage or stages thereafter, thus imaginary short is established between the inverted and non-inverted input terminals of the differential pair which composes the first stage. As the result, a potential difference applied between the input terminals becomes very small, for example, about several millivolts. Accordingly, the linearity of the differential pair is hardly impaired in this case.
While, differential pairs are used for filters, multipliers, oscillators and the like because the transconductance G.sub.m can be changed in proportion to the operational current. In such cases, it is ordinarily desired that a voltage applied between the input terminals of the differential pair be large within the range of the linear operation because of the S/N ratio and the like. Thus, a wider linear operational range is required in order to handle an input signal of a greater level.
However, as shown in FIG. 2B, in the conventional differential pair, a flat portion of the curve of the transconductance Gm in the vicinity of Vd=0 is very narrow, and the range of Vd where an absolute value of Gm is reduced by 1% from the maximum value is about 10 mV at ordinary temperature.
The it is also known that there is a method called emitter degeneration for enlarging the linear area by connecting between emitters of differential pairs through resistance to apply a local negative feedback rather than connecting them directly. This method is simple and effective, however, noise is increased because of the resistance. Also it is difficult to change the transconductance because of the negative feedback. Thus, there are occasional drawbacks when applying this method to filters and the like.
Moreover, there is a method using a differential amplifier of a Gilbert gain cell type as a means for changing a transconductance Gm by using the emitter degeneration. Since the gain cell itself is described in detail in pp. 590-600 of the literature [A] or in the following literature, the detailed explanation is not given here.
LITERATURE: "BIPOLAR AND MOS ANALOG INTEGRATED CIRCUIT DESIGN", ALAN B. GREBENE, John Wiley & Sons, New York, 1984, pp. 437-443.
In short, a first differential pair has an emitter degeneration resistance, and a transistor whose base and collector are shorted is provided as the load, and the potential difference between both the terminals is inputted to a second differential pair not having the emitter degeneration resistance. As the result, by changing a current of the common emitter of the second differential pair, the transconductance from the base input terminal of the first differential pair to the collector output terminal of the second differential pair can be changed.
Moreover, the differential amplifier using the gain cell can easily realize a linear input range of 1 volt. As an example of composing a filter using such a linearized differential amplifier, there is "Multiple purpose filter" disclosed in Japanese Patent Application for Disclosure No. 58-161413.
However, according to the method, since compression and expansion of the signal voltage are carried out to cancel the exponential characteristics of the transistor, the S/N ratio is more deteriorated than that of a simple differential pair even through the linear range can be enlarged.
To solve the problem, there is a proposition for linearizing the differential pair without using the emitter degeneration discussed in literature [B].
Literature [B]: James C. Schmoock; "An Input Transconductance Reduction Technique for High-Slew Rate Operational Amplifiers", IEEE Journal of Solid-State Circuit, SC-10, no. 6, pp. 407-411, December 1975.
This proposition mainly refers to a method of reducing the transconductance by using two differential pairs whose emitter areas are asymmetrical to the fact each other, and also refers to that the linear operation range becomes the widest when the ratio of the emitter areas is about 1:4.
However, according to the method, though the linear range can be enlarged about four times as compared with that of the conventional simple differential pair, it is still insufficient. Since the input terminal is directly the base of the transistor, the input impedance is relatively large.
Moreover, in order to obtain a wide linear operation range, there is a proposition to linearize the differential pair without using the emitter degeneration in "Transconductance amplifier" of Japanese Patent Application for Disclosure No. 62-200808. The method can provide an extremely wide linear range substantially equal to that of the linearized differential amplifier with the gain cell, and can guarantee a preferable S/N ratio.
The principle of the method is, in short, to operate the differential pair in class "AB" operation. It is, therefore necessary to give an operational current corresponding to the input voltage. To realize this operation, the input voltage is divided by resistances and given to bases of a plurality of transistors. However, since these resistances are inserted in series to the bases, it is unpreferable to set them at relatively large values in view of noise and frequency characteristics. Accordingly, to utilize the feature of the circuit, it is necessary that the input resistances be relatively low.
The restriction is not preferable for the circuit design. For example, since an integrator is composed by connecting a capacitor to the output terminal of the transconductance amplifier, a filter can be composed of mutual connections of a plurality of the integrators.
In such a configuration, since an input terminal of one integrator is connected to an output terminal of another integrator, a low input resistance of one integrator is loaded on the output terminal of another integrator, so that the Q value (Quality factor) of the filter is makedly lowered.
As stated above, the following problems still remain in the conventional differential pair or circuit.
(1) A differential amplifier whose linear operation range is wide and transconductance is changeable is required for filters, multipliers and oscillators. However, though the conventional differential circuit of the gain cell type using the emitter degeneration has a wide linear operation range, the S/N ratio thereof is inferior.
(2) While, the class "AB" operation circuit not using the emitter degeneration has a wide linear range and a preferable S/N ratio, the input impedance is low.